Hi Steve.

You may be having a problem with some extension or plugin that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check), to see if Firefox goes back to normal?

A possible cause is security software (firewall) that blocks or restricts Firefox without informing you about that, maybe after detecting changes (update) to the Firefox program. Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox.

See Configure firewalls so that Firefox can access the Internet and http://kb.mozillazine.org/Firewalls

See also http://kb.mozillazine.org/Browser_will_not_start_up